This Week: Biogeochemical Cycles
Hydrologic Cycle
Carbon Cycle
Announcements
• Reading: Chapters 4 (p. 74 – 81) and 8
• Another Problem Set (Due next Tuesday)
• Exam 2: Friday Feb 29
• My office hours today and next week
rescheduled to Thurs 3 - 4 pm
Water Cycle “Box Model”
Burden:
amount of
material in
reservoir
Source/Sink: flow
rate into/out of
reservoir
Reservoir: region where
material stored; each box
There’s ~ 1.3x1018 m3 of H2O in the oceans.
About 3.6x1015 m3 of H2O per year evaporate
from the oceans. How long does the average H2O
molecule reside in the ocean?
1. 36 years
2. 360 years
3. 3600 years
61%
35%
ar
s
36
00
ye
ye
a
0
36
36
ye
ar
rs
s
5%
Precipitable Water Distribution
Precipitable water greatest over oceans, in tropics.
Spatially Resolved Precipitation Rate
Subsiding branches of
Hadley Cells
Precipitation Rate Minus Evaporation Rate
green (positive): net water gain
yellow/brown (negative): net water loss
Water Cycle and Terrestrial Biosphere
Precipitation
Forest
Grassland
Tundra
Desert
0o C
Temperature
Precipitation Changes with Global Warming
Climate Model Predictions
Announcements
• Problem Set 4 Due Tuesday Feb 26
• Group Choices by Friday
• Seminar on Thursday 12:30 310c ATG
Carbon Cycle
• Short-term biosphere-driven cycle
– Terrestrial biosphere
– Marine biosphere
• Long-term inorganic cycle
– Weathering-volcanism thermostat
Global Biogeochemical Cycles
Reduced gas
EARTH
SURFACE
Oxidation
Emission
Oxidized gas/
particle
Uptake
Reduction
Deposition
Atmospheric CO2 Seasonal Cycle
CO2 Mixing Ratio (ppm) at Mauna Loa
350
348
Monthly Mean CO2 Mixing Ratio
346
344
342
340
338
336
334
332
330
1980
1981
1982
1983
1984
1985
Time
Atmosphere responds to biosphere on short timescales
Mass Units of Geologic Scale
• 1 Gigaton = 1x109 tons = 1x1012 kg =
1 x 1015 grams = 1 Petagram
• 1 Gigaton = 1 Petagram
Terrestrial Biosphere C Cycle
Flows in units of
Gtons C per year
Atmosphere
780 Gtons
NPP: 60
Living Biomass
750 Gtons
Decay/resp: 49
52
Litter
110 Gtons
Decay/resp: 11
8
Steady State?
Longest lifetime?
Shortest lifetime?
3
Soil
2000 Gtons
The effect of deforestation on
atmospheric CO2 is
94%
1. to increase it (source)
2. to decrease it (sink)
3. negligible
5%
gl
ig
ib
ne
as
e
cr
e
de
to
le
)
it
(s
in
k
)
ou
rc
e
(s
it
se
re
a
in
c
to
2%
Under conditions of higher CO2 (and
sufficient nutrient supply), plants have been
shown to increase photosynthesis rates.
58%
1. Positive feedback
2. Negative feedback
3. Not a feedback
27%
k
db
ac
fe
e
a
ot
N
at
iv
e
eg
N
Po
si
tiv
e
fe
fe
e
db
ed
ba
ac
k
ck
15%
Marine Organic C Cycle
CO2
Surface Ocean
Deeper Ocean
Photosynthesis
decay
CO2(aq) + nutrients
death/fecal matter
OrgC sinks
OrgC and
nutrients
O2
CO2(aq)
a tiny bit to sediments
Biological OrgC Pump: Key Points
1. Surface depleted (relatively) in C and nutrients
2. Deep ocean enriched in C and nutrients
3. Atmospheric CO2 responds to net pumping
CO2 and nutrients
pumped down by biota
Surface Ocean
Thermo-Haline
Circulation
~1000 yrs
Deeper Ocean
OrgC + O2
CO2(aq)
Marine Productivity
Global map of ocean color from SEAWIFS satellite
chlorophyll phytoplankton (where the nutrients are)
remember upwelling and convection?
Reservoirs of (Inorganic) Carbon
Atmosphere
790 Gtons
Mainly as CO2
Ocean
3.7x104 Gtons
Mainly as HCO3-
Lithosphere
4x107 Gtons
Mainly as CaCO3
Carbon in the oceans is mostly in an inorganic form
Shell Formation
CO32-
+
Slow
THC
DIC
Ca2+
Shell formation
(CaCO3(s))
Surface Ocean
death/shells sink
slow
CaCO3(s)
Deep Ocean
Small amnt to sediments
Carbonate shell formation ___________ the
ocean’s capacity to take up atmospheric CO2
1. increases
2. decreases
82%
s
se
cr
ea
de
in
cr
ea
se
s
18%
The “Ultimate” Inorg C Cycle
CaSiO3 + CO2
CaCO3 + SiO2
Silicate Weathering
CaSiO3 + CO2  CaCO3 + SiO2
Rain/runoff
CO2 (atm)
CaSiO3 Rock
CaCO3 + SiO2
CaSiO3 Rock
Chemical Weathering Rate
Faster with higher CO2,
higher T, higher rainfall
Ca2+
CO32SiO2
Oceans
Volcanic Degassing
Volcanism causes reverse of weathering
CaSiO3 + CO2
HEAT
CaCO3 + SiO2
Tectonic activity converts CaCO3 rocks back to
silicate rocks in the mantle (magma).
CO2 released finds way to atmosphere via vents
Silicate Weathering “Thermostat”
CO2
Chemical weathering
CaSiO3  CaCO3 + SiO2
Precip/runoff
CaCO3 + SiO2
burial
CaCO3 + SiO2
CaCO3 + SiO2  CaSiO3 + CO2
This cycle operates on 0.5 – 1 million year timescale.
Is the silicate weathering – volcanism InorgC
cycle a positive or negative feedback?
1. Positive
2. Negative
58%
at
iv
e
eg
N
Po
si
tiv
e
42%
Silicate Weathering Feedback
Negative Feedback—Stabilizing Climate
-
+
Initial Forcing
Silicate
Weathering Rate
CO2
+
+
Temperature/
Precipitation
Does the silicate weathering feedback loop
explain the glacial-interglacial cycling of
atmospheric CO2?
55%
o
N
s
45%
Ye
1. Yes
2. No